1
What is JavaScript? Easy
JavaScript is a high-level interpreted programming language used to create dynamic and interactive web applications.
2
What are JavaScript data types? Easy
Primitive types include string, number, boolean, null, undefined, symbol, bigint. Non-primitive type is object.
3
What is undefined? Easy
A variable declared but not assigned a value is undefined.
4
What is null? Easy
Null represents an intentional absence of value.
5
Difference between var, let, and const? Easy
var is function scoped. let and const are block scoped. const cannot be reassigned.
6
What is hoisting? Easy
JavaScript moves variable and function declarations to the top of their scope during compilation.
7
What is a closure? Easy
A closure is a function that remembers variables from its outer lexical scope.
8
What is the DOM? Easy
DOM is the Document Object Model that represents HTML structure as objects.
9
Difference between == and ===? Easy
== performs type coercion. === checks both value and type.
10
What is an array? Easy
An array is an ordered collection of values.
11
What is an object? Easy
An object is a collection of key-value pairs.
12
What is a function? Easy
A function is a reusable block of code.
13
What is JSON? Easy
JSON is JavaScript Object Notation used for data exchange.
14
What is NaN? Easy
NaN means Not a Number.
15
What is template literal? Easy
Template literals allow string interpolation using backticks.
16
What is an arrow function? Easy
Arrow function is a shorter syntax for writing functions.
17
What is this keyword? Easy
this refers to the execution context object.
18
What is event bubbling? Easy
Event bubbling propagates events upward in DOM tree.
19
What is event capturing? Easy
Event capturing propagates events downward in DOM tree.
20
What is localStorage? Easy
localStorage stores data permanently in browser.
21
What is sessionStorage? Easy
sessionStorage stores data for current session only.
22
What is type coercion? Easy
Automatic conversion of data types.
23
What is strict mode? Easy
Strict mode enforces stricter parsing and error handling.
24
What is a callback function? Easy
Function passed as argument to another function.
25
What is recursion? Easy
Function calling itself repeatedly.
26
What is spread operator? Easy
Spread operator expands iterable using ...
27
What is destructuring? Easy
Extract values from arrays or objects.
28
What is Promise? Easy
Promise represents future completion of async operation.
29
What is map() method? Easy
map() creates new array by transforming each element.
30
What is filter() method? Easy
filter() returns elements that match condition.
31
What is reduce()? Easy
reduce() reduces array to single value.
32
Difference between null and undefined? Easy
null is intentional absence; undefined means not assigned.
33
What is typeof operator? Easy
typeof returns data type of variable.
34
What is IIFE? Easy
Immediately Invoked Function Expression executes immediately.
35
What is event listener? Easy
Event listener waits for specific event to occur.
36
What is setTimeout? Easy
setTimeout executes function after delay.
37
What is setInterval? Easy
setInterval repeatedly executes function.
38
What is ES6? Easy
ES6 is ECMAScript 2015 version introducing modern features.
39
What is default parameter? Easy
Default value assigned to function parameter.
40
What is rest parameter? Easy
Rest parameter collects multiple arguments into array.
41
Explain Event Loop in detail. Medium
Event Loop continuously checks call stack and task queues to execute asynchronous tasks without blocking main thread.
42
What is Call Stack? Medium
Call stack is a data structure that keeps track of function execution.
43
Microtasks vs Macrotasks? Medium
Microtasks (Promises) execute before macrotasks (setTimeout).
44
Explain lexical scope. Medium
Lexical scope means scope is determined by where variables are written in code.
45
What is closure in loops issue? Medium
Using var in loops creates shared reference causing unexpected behavior.
46
Explain prototype chain. Medium
Objects inherit properties through prototype chain lookup.
47
Difference between deep and shallow copy? Medium
Shallow copy copies references. Deep copy clones nested objects fully.
48
What is Object.freeze()? Medium
Prevents modification of object properties.
49
What is Object.seal()? Medium
Prevents adding/removing properties but allows modification.
50
What is currying? Medium
Transforming a function into a sequence of functions taking one argument.
51
Explain bind(), call(), apply(). Medium
Used to explicitly set this context.
52
What is event delegation? Medium
Handling events at parent level instead of individual elements.
53
What is debouncing? Medium
Limits execution of a function until delay passes.
54
What is throttling? Medium
Ensures function executes at fixed intervals.
55
Explain Promise chaining. Medium
Allows sequential execution using .then().
56
What is async/await internally? Medium
Syntactic sugar over Promises.
57
How fetch() works? Medium
Fetch returns Promise resolving to Response object.
58
What is CORS? Medium
Cross-Origin Resource Sharing policy controlling resource access.
59
What is WeakMap? Medium
WeakMap holds weak object references allowing garbage collection.
60
What is WeakSet? Medium
WeakSet stores unique objects with weak references.
61
Explain Symbol data type. Medium
Symbol creates unique identifiers.
62
What is generator function? Medium
Function* that yields multiple values lazily.
63
What is Map vs Object? Medium
Map maintains insertion order and allows any key type.
64
Explain destructuring nested objects. Medium
Extract nested properties using pattern matching.
65
What is spread vs rest? Medium
Spread expands, rest collects values.
66
What is module pattern? Medium
Encapsulates private variables using closures.
67
Explain Immediately Invoked Async Function. Medium
Async IIFE executes immediately returning Promise.
68
Explain event bubbling vs capturing. Medium
Bubbling flows up, capturing flows down DOM tree.
69
What is tree shaking? Medium
Removes unused code during bundling.
70
Explain dynamic imports. Medium
Load modules on demand using import().
71
How to prevent default behavior? Medium
Use event.preventDefault().
72
Explain memoization. Medium
Caching function results for performance.
73
Explain tail call optimization. Medium
Optimizes recursive calls to avoid stack overflow.
74
Explain strict mode advantages. Medium
Catches silent errors and prevents unsafe actions.
75
What is optional chaining? Medium
Safely access nested properties using ?.
76
Explain nullish coalescing. Medium
Returns right operand if left is null or undefined.
77
How does JSON.parse work? Medium
Converts JSON string into JS object.
78
What is error handling in async? Medium
Use try/catch with async await.
79
What is service worker? Medium
Runs in background enabling offline support.
80
Explain browser rendering process. Medium
HTML → DOM → CSSOM → Render Tree → Layout → Paint.
81
Explain memory heap. Medium
Heap stores objects in memory.
82
What is IIFE privacy pattern? Medium
Encapsulates variables avoiding global scope pollution.
83
Explain module bundlers. Medium
Tools like Webpack bundle JS modules.
84
What is event loop starvation? Medium
Long sync tasks blocking async queue.
85
Explain structured cloning. Medium
Deep copying using structuredClone().
86
What is proxy trap? Medium
Intercepts object operations using Proxy.
87
Explain memory leaks in closures. Medium
Unreleased references prevent GC.
88
How to handle large lists efficiently? Medium
Use virtualization and pagination.
89
What is browser repaint vs reflow? Medium
Reflow recalculates layout, repaint updates visuals.
90
Explain WebSockets. Medium
Full duplex communication protocol over TCP.
91
How V8 engine works internally? Hard
V8 uses JIT compilation converting JS to machine code.
92
Explain Just-In-Time compilation. Hard
Compiles code at runtime for performance.
93
What is garbage collection algorithm? Hard
Mark-and-sweep algorithm cleans unused memory.
94
Explain event loop phases in Node.js. Hard
Timers → Pending → Idle → Poll → Check → Close callbacks.
95
Implement custom Promise. Hard
Create class with then/catch methods managing state.
96
Explain microtask queue priority. Hard
Microtasks run before next macrotask.
97
How to detect memory leak? Hard
Use Chrome DevTools heap snapshots.
98
Explain Shadow DOM internals. Hard
Encapsulated DOM subtree.
99
What is CSP? Hard
Content Security Policy prevents XSS attacks.
100
How to prevent XSS? Hard
Escape user input and use CSP.
101
Explain SSR vs CSR. Hard
Server-side vs client-side rendering.
102
What is WebAssembly? Hard
Binary format enabling near-native speed execution.
103
Explain concurrency model. Hard
Single-threaded with async event-driven architecture.
104
How prototype inheritance differs from classical? Hard
JS uses delegation model.
105
Explain functional programming concepts. Hard
Pure functions, immutability, higher-order functions.
106
Implement debounce function manually. Hard
Use setTimeout clearing previous timer.
107
Implement throttle manually. Hard
Use timestamp or flag to limit calls.
108
Explain virtual DOM concept. Hard
Efficient DOM diffing before updates.
109
What is event loop blocking? Hard
Long sync tasks block async tasks.
110
Explain heap vs stack memory. Hard
Stack stores primitives, heap stores objects.
111
How to optimize rendering? Hard
Minimize DOM operations and reflows.
112
Explain web worker limitations. Hard
No direct DOM access.
113
Explain atomic operations in JS. Hard
Used with SharedArrayBuffer.
114
What is SharedArrayBuffer? Hard
Shared memory between workers.
115
Explain streaming APIs. Hard
Process data chunk by chunk.
116
What is IndexedDB? Hard
Low-level API for client storage.
117
Explain event delegation performance benefit. Hard
Reduces number of listeners.
118
Explain lazy loading modules. Hard
Load modules only when needed.
119
Explain tree shaking in depth. Hard
Static analysis removes unused exports.
120
How to build large scale JS app? Hard
Use modular architecture, state management, code splitting.
📊 Questions Breakdown
🟢 Easy
40
🟡 Medium
50
🔴 Hard
30
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